This invention relates to a device for measuring various characteristics of a color cathode ray tube (hereinafter referred to as color CRT) by picking up an image displayed on a faceplate of a color CRT.
The color CRT has a faceplate whose inner surface is covered with regularly arranged deposits of red, green and blue phosphor materials that emit light to form a color image when hit by the electron beams for the respective colors according to primary color video signals. In a color CRT production line, various characteristics of the color CRT, e.g., misconvergence, is measured to obtain desired performances of the color CRT. Specifically, a test pattern is displayed on the CRT faceplate and an image signal is obtained by picking up an image of the displayed test pattern to measure a characteristic of the CRT.
A conventional measurement device includes a measuring main body and image pickup means, and picks up an image of a test pattern displayed on the CRT by the image pickup means. The image pickup is carried out by the following steps, for example. A head of the image pickup means is held in contact with a selected pickup region of the CRT faceplate. A focusing ring is manually rotated so that the lens of the image pickup means focuses on the target image displayed on the inner surface of the CRT faceplate while observing an image displayed on a monitor screen. After being focused, the image pickup means picks up the image of the test pattern for measurement.
The conventional measurement device requires the manual focus adjustment of the image pickup means, involving complicated image pickup operations and long measurement time. Also, the focusing state is checked by observing an image displayed on the monitor screen with operator's eyes. Consequently, the accuracy of focus adjustment is not constant, resulting in accumulated errors in repeated measurements.
There have been proposed measurement devices provided with an autofocus system to make the focusing of the image pickup means automatic. For example, there has been proposed a measurement device provided with an autofocus system in the fashion of the trigonometrical range measuring in which: a light beam is projected upon an object from an emitter and a reflected light is received by an optical sensor apart a reference length from the emitter; the distance to the object is calculated based on the received position; and the lens is driven in accordance with a calculated distance. Also, there has been proposed a measurement device provided with an autofocus system in the fashion of the phase difference detection in which: object light passed through an objective lens is separated into two focused images by two separator lenses; a phase difference between the two focused images is detected; and the objective lens is driven so as to eliminate the phase difference.
However, in the measurement device with the trigonometrical range measuring type of autofocus system, the light beam reflects at an outer surface of the faceplate. Consequently, the lens is focused on the outer surface of the faceplate, not exactly focused on the inner surface of the faceplate bearing the test pattern.
Also, in the measurement device with the phase difference detection type of autofocus system, it is practically impossible to detect a phase difference because of the fact that: the test pattern consists of fixedly repeated images; the phase difference cannot consequently be detected.
In measurement devices provided with an autofocus system, further, it is critical what portion of the pickup region of the image pickup means is focused on. It is usual practice to focus the image pickup means on a center of the pickup region. In this center focusing, however, if an image at an end portion of the pickup region is selected for measurement, the selected image will be out of focus, and provide a wrong measurement. This problem is keen in measuring a misconvergence of a CRT having a curved faceplate because there is a considerable difference between the distance to an image at a center portion of the pickup region and that to an image at and portion of the pickup region.
Further, in the focus adjustment based on light from an image on the faceplate of a CRT, there is a likelihood that only an upper or lower half of a target image is picked up for an improper pickup timing. In such a case, the autofocusing will be impossible because of insufficient light.
As an example of misconvergence measurement device, Japanese Unexamined Patent Publication No. 2-174492 discloses a misconvergence measurement device which provides a white crosshatch test pattern on the faceplate of a color CRT, picks up an image having an intersection portion of the white crosshatch test pattern to produce image signals for red, green, and blue images of the picked up image respectively, and measures misconvergences in horizontal and vertical directions based on the red, green and blue image signals.
When using such a white crosshatch test pattern, an image pickup means of the misconvergence measurement device is required to be positioned so that the pickup region of the pickup means covers an intersection portion of the crosshatch test pattern and both horizontal and vertical lines. In conventional measurement devices, such positioning of image pickup means is carried out by hands of the operator as well as focusing of image pickup means, which consequently makes it difficult to accomplish easy and speedy misconvergence measurement.
Specifically, when an intersection portion of the crosshatch pattern is located at an end portion of the pickup region, proper misconvergence measurement cannot be carried out and the image pickup means is accordingly required to be moved so that the intersection portion comes into a center of the pickup region. This is a cumbersome operation.
Also, it is difficult to visually check on a monitor screen if the image pickup means is located at an appropriate position. Accordingly, this will make difficult prompt positioning of the image pickup means. In the case of measuring misconvergences at a plurality of points on the faceplate, particularly, a considerably longer time is consumed for misconvergence adjustment. This will lower the productivity of color CRTs.
For measurements other than misconvergence measurement, it i s necessary to move the image pickup means so that an image of the test pattern comes into a specified position of the pickup region if not at the specified position. Similarly to the above, this will make the measurement time longer.
It is possible that an intersection portion of the crosshatch pattern is always within the pickup region by reducing the mesh pitch of the crosshatch pattern. This will eliminate the necessity of moving the image pickup means for positioning. In a crosshatch pattern of a reduced mesh pitch, however, there is a likelihood that in misconvergence measurement, a target line cannot be discriminated from adjacent lines.
To accurately measure a characteristic of a color CRT, further, it is necessary to hold the image pickup means in such a position that an optical axis of the lens system of the image pickup means is perpendicular to the faceplate of the CRT, in other words, to hold the image sensing plane of the image pickup means parallel to the CRT faceplate. However, it is very difficult to achieve this positioning because of the curvature of the CRT faceplate. In practice, consequently, it has been unavoidable to pick up a target image in an oblique direction and result in measurement errors.